Game mechanics isn't a single tool, but a set of tools. Before you pick which tool you're going to use, you need to decide what problem you're trying to solve.

If your goal is to increase activity, then this is too broad. Who do you want activity from? And why do you want activity from them? What sort of activity do you want from them?

Here are the different tools you can use to rank members and the benefits you gain from each.

By access levels. What is the members access level to the site? Can they access areas of the community that other members can't? Can they remove the negative posts without having to point them out? Can they create content without approval? This is the usually the ultimate level of trust and authority in a community. You usually want to bestow these honours upon members individually rather than automate the system.

By registration date. Rank members by how long they have been registered members of the site. Useful for pleasing veteran members of the community. If you have been losing veterans, this is a great tool to use. It shows a clear line between the vetarans and the newcomers.

By quantity of contributions. Rank members by the number of contributions they have made to the community. The forum/post count is a default setting in most platforms. It encourages contributions, but can encourage too many contributions without regard to quality. If members aren't making enough contributions, emphasizing this can increase the level of contributions. It's usually best used with another

By quality of contributions. My favourite. Rank members by the reaction to their contributions. How many thumbs up/thumbs down have they received on the topic? How many positives have their contributions made to the community? What do other members think of this member? This encourages all members to improve the quality of contributions (and rate others to induce reciprocity – but this is a good thing, it enhances the level of interactions between members).

By frequency. Rarely used, but rank people by how often they visit the community. Have they visited the community 17530 times? Or do they average 177 visits per week? This can be a fantastic way to get people to visit more often. It will be gamed, but that doesn't matter if more people are visiting more often. This leads to more participation.

Special distinction. Give members by specially awarded distinctions. You individually add a distinction to their profile. It's unique and no other member can acquire it. I love these. It gives you power to reward and encourage specific types of contributions from specific members at specific times. You can bestow expert status upon a member of a very niche area in the community.

Leaderboards. Rank members by how they stand overall amongst other members of the community. Scoreboards are the typical standard here. This encourages competition between members.

By a combination of the above. Use a Kloutish score. Combine several of the elements above into one individual score that members have.

Refrain from tying your game mechanic systems to a prize. The prize should be the respect and enhanced reputation they gain from members seeing how highly they are ranked. If you explicitly link these scores to a stated prize, the whole system becomes meaningless. Members will attempt to game the system and if you ever remove the prizem, the level of participation will suddenly drop.